Implantable medical devices (IMDs) such as implantable pacemakers, implantable cardioverter defibrillator (ICD) devices, cardiac resynchronization therapy defibrillator devices, can detect and deliver therapy for a variety of medical conditions in patients. ICDs typically comprise, inter alia, a control module, a capacitor, and a battery that are housed in a hermetically sealed container with a lead extending therefrom. When therapy is required by a patient, the control module signals the battery to charge the capacitor, which in turn discharges electrical stimuli to tissue of a patient through via electrodes disposed on the lead, e.g., typically near the distal end of the lead.
The control module typically includes one or more hermetically encased integrated circuits (ICs) or chips, which is a miniaturized electronic circuit. An IC is comprised of semiconductor devices (e.g. diode, transistor etc.) and passive components (e.g., transistors, capacitors, resistors, etc.) that are formed in the surface of a thin substrate of semiconductor material. One IC can be connected to another IC or other wafer through wafer to wafer bonds. Wafer to wafer bonds relates to joining major surfaces of the wafers. The joined areas of the wafers creates the hermetic seal(s).
A typical wafer to wafer bond relies on a copper pad disposed on each wafer. The copper pad is higher than the surrounding plane of the wafer. A copper pad on one wafer is aligned with the copper pad on the other wafer. Thermo-compression diffusion bonding can be employed to join the copper pads located on each wafer. The ICs are then sealed together with a copper seal ring or a race track near the outer edges of the individual chips. Copper is not biostable and may not provide an adequate seal in vivo. Additionally, copper pads that are coplanar with a thermal oxide can be difficult to planarize and polish. For example, copper and thermal oxide can have different polishing rates. It is therefore desirable to develop new techniques for efficiently and hermetically sealing the electronic circuitry in IMDs. Additionally, a portion of an alloy may need to removed from a substrate before formation of a wafer to wafer bond. Some alloys can only be removed to a certain level and cannot attain a certain level of removal. It is therefore desirable to develop new techniques for efficiently and hermetically sealing the electronic circuitry in IMDs.